Tell them you love them when you are absolutely sure that you do, and when you are certain you are willing to commit to this relationship for a longer time.
No need to rush it, especially if they have not said anything along those lines yet either.
Edit: As for "When do I know that I truly love them" I'd say that's the case when you not only feel attracted to them, but also actually care about their wellbeing. When it hurts you to see them hurting, or when it brings you joy when they are happy, that's probably love.
Tell them you love them when you are absolutely sure that you do, and when you are certain you are willing to commit to this relationship for a longer time.
No need to rush it, especially if they have not said anything along those lines yet either.
Edit: As for "When do I know that I truly love them" I'd say that's the case when you not only feel attracted to them, but also actually care about their wellbeing. When it hurts you to see them hurting, or when it brings you joy when they are happy, that's probably love.
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